Thursday, November 21, 2013

Nano Dots meets Solar Cells

Researchers at National University of Singapore and Nanyang Technological University, Singapore, have demonstrated the use of nanodots to increase the efficiency of solar cells. In today's commercial solar cells between 17% to 19% of the photons hitting the cells get converted into electricity. With the advent of large giga scale solar projects an increase in efficiency of a few percentage points could lead to huge gains for the industry.
The team from Singapore attacked the problem using an effect known as upconversion. Our sun generates an enormous amount of energy in the form of photons of light. Solar Cells absorb the energy from photons which have a certain threshold of energy and create electrons or electricity from them. However, not all photons coming from our sun have the same energy. The lower energy photons are not absorbed by the solar cells and are lost. Researchers were able to combine two low energy photons to produce a single high energy photons which could then be absorbed by the solar cell to generate electricity.
The team used a
structure made of titanium oxide called an inverse opal (see above).
The
inverse opal frame is filled with an arrangement of air pores roughly
half a micrometer across. Nanospheres, about 30 nanometers in diameter
converted lower energy photons (NIR - near infrared radiation) to
higher energy photons of visible light. The opal frame is coated with
light sensitive cadmium selenide quantum dots which absorb the photons
to release electrons which are then transmitted through the titanium
oxide frame.
The researchers tested the device using laser light with a wavelength of 980 nanometers. This wavelength is not normally absorbed by cadmium selenide quantum dots. The experiment yielded much higher electric current as compare to devices not using the upconversion material. The researchers believe their upconversion based device could yield a significant competitive advantage over conventional silicon solar cells. More on this is available on the Lab's website.
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